1. Field of the Invention
The present invention generally relates to telescopic sights as used on rifles, cameras, other bore sighted equipment or alone for surveillance. In particular it is concerned with electronic night sight modules for use with day sights using imagers or image intensifiers that convert infrared radiation to visible light.
2. Description of Prior Art
Under bright sunlight conditions the telescopic rifle sight is an excellent, low cost solution for selecting targets at great distances. Under less favorable conditions such as heavy overcast, moonlight or starlight an electronic sight using an image intensifier has been used in place of this sight. The light is sensed by a photoemmitter, which is far more sensitive than the eye, and this responds by generating free electrons. These electrons are then multiplied and used to activate a light emitting phosphor. The Photoemitter not only requires less light intensity, but is also sensitive to infrared and ultraviolet that the eye does not see. Similar solid state devices can be made using photodiodes and light-emitting diodes, but these become complicated and generally do not provide as much intensification. Diode devices have also been developed for far-infrared. These produce specialized temperature images, which may not be recognizable, and also require cooling to cryogenic temperatures.
Attempts have been made to combine day and night sights to obtain the advantages of both. An early suggestion was to use a telescopic sight that provided a different image to each of the user's eyes. The user's brain was supposed to combine the two into a single image. This proved to be very disturbing to the user and was discontinued. Separate sights used interchangeably is the generally preferred procedure at this time.
Another approach is illustrated in the U.S. Pat. No. 4,629,295 "NIGHT VISION INSTRUMENT WITH ELECTRONIC IMAGE CONVERTER" granted Dec. 16, 1986 to Georg Vogl. The teachings of this patent have been translated into a night sight module that attaches to the objective end of a day sight. This is identified as the SIMRAD model KN200/250 optical system. While this system does provide a day/night capability, it does not efficiently utilize the capability of its image intensifier tube.